Semiconductor thin film and semiconductor device

ABSTRACT

After an amorphous semiconductor thin film is crystallized by utilizing a catalyst element, the catalyst element is removed by performing a heat treatment in an atmosphere containing a halogen element. A resulting crystalline semiconductor thin film exhibits { 110 } orientation. Since individual crystal grains have approximately equal orientation, the crystalline semiconductor thin film has substantially no grain boundaries and has such crystallinity as to be considered a single crystal or considered so substantially.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor thin film formed on asubstrate having an insulating surface and a semiconductor device usingsuch a semiconductor thin film as its active layer. In particular, theinvention relates to such a semiconductor thin film and semiconductordevice in which the semiconductor thin film is made of a material havingsilicon as the main component.

Further, the invention relates to a semiconductor circuit and anelectro-optical device that are constituted of semiconductor devicessuch as thin-film transistors as well as to an electronic apparatususing such a semiconductor circuit and electro-optical device.

In this specification, the term “semiconductor device” is used asincluding all of the above-mentioned thin-film transistor, semiconductorcircuit, electro-optical device, and electronic apparatus; that is, allof devices and apparatuses that function by utilizing a semiconductorcharacteristic are called semiconductor devices. Therefore, thesemiconductor devices recited in the claims are not limited to singleelements such as a thin-film transistor and encompass semiconductorcircuits and electro-optical devices that are constructed by integratingsuch single elements as well as electronic apparatuses using such asemiconductor circuit or electro-optical device as a part.

2. Description of the Related Art

In recent years, the techniques of forming thin-film transistors (TFTs)by using a semiconductor thin film (thickness: tens to hundreds ofnanometers) formed on a substrate having an insulating surface haveattracted much attention. Thin-film transistors particularly asswitching elements of image display devices such as liquid crystaldisplay devices are now being developed at high speed.

For example, in liquid crystal display devices, it is attempted to applyTFTs to every kind of electric circuit such as a pixel matrix circuit inwhich pixel regions arranged in matrix form are controlled individually,a driver circuit for controlling a pixel matrix circuit, or a logiccircuit (an operation circuit, a memory circuit, a clock generator, orthe like) for processing an external data signal.

At present, TFTs using an amorphous silicon film as an active layer havebeen put into practical use. However, TFTs using a crystalline siliconfilm such as a polysilicon film are necessary for electric circuits,such as a driver circuit and a logic circuit, that are required tooperate at even higher speed.

For example, techniques of the present assignee that are disclosed inJapanese Laid-open Patent Publication Nos. Hei. 7-130652 and Hei.8-78329 are known as methods for forming a crystalline silicon film on aglass substrate. The disclosures of which are incorporated herein byreference. By utilizing a catalyst element for acceleratingcrystallization of an amorphous silicon film, the techniques of thesepublications enable formation of a crystalline silicon film havingsuperior crystallinity by a heat treatment of 500°-600° and about 4hours.

In particular, the technique of the publication No. 8-78329 is such thatthe above technique is utilized to cause crystal growth in a directionapproximately parallel with the substrate surface. The present inventorsespecially call a resulting crystallized region a “lateral growthregion.”

However, even a driver circuit that is constructed by using such TFTscannot completely provide required performance. In particular, atpresent, it is impossible to construct, by using conventional TFTs,high-speed logic circuits that are required to operate at extremely highspeed (megahertz to gigahertz).

To improve the crystallinity of a crystalline silicon film having grainboundaries (called a polysilicon film), the inventors have repeatedtrial and error as exemplified by a semi-amorphous semiconductor(Japanese Laid-open Patent Publication No. Sho. 57-160121 etc.) and amonodomain semiconductor (Japanese Laid-open Patent Publication No. Hei.8-139019). The disclosures of which are incorporated herein byreference.

SUMMARY OF THE INVENTION

The concept common to the semiconductor films described in the abovepublications is to make grain boundaries substantially harmless. Thatis, the most important object was to substantially eliminate grainboundaries to thereby enable smooth movement of carriers (electrons orholes).

However, the semiconductor films described in the above publications arestill insufficient to allow logic circuits to perform requiredhigh-speed operation. That is, to realize a system-on-panelincorporating logic circuits, it is necessary to develop a material thatis not known, i.e., an entirely new material.

An object of the present invention is to satisfy the above requirement,that is, to provide a semiconductor thin film capable of realizing asemiconductor device having extremely high performance that allowsconstruction of such a high-speed logic circuit as conventional TFTscannot provide. Also, another object of the present invention is toprovide a semiconductor device using the semiconductor thin filmdescribed above.

The invention provides a semiconductor thin film which is a collectedbody of a plurality of rod-like or flat-rod-like crystals each havingsilicon as the main component, wherein the main orientation planeapproximately coincides with the {110} plane; the concentration, in thesemiconductor thin film, of each of carbon and nitrogen is 5×10¹⁷atoms/cm³ or less and the concentration of oxygen is 1×10¹⁸ atoms/cm³ orless; and the rod-like or flat-rod-like crystals contact each otherwhile forming rotation angles having absolute values that are within 3°.

The above semiconductor thin film may be such that an electron beamdiffraction pattern of the semiconductor thin film has particularregularity due to {110} orientation, that each of diffraction spots ofthe electron beam diffraction pattern is approximately circular, andthat the ratio of the minor-axis length to the major-axis length of eachof the diffraction spots is in a range of 1/1 to 1/1.5.

The above semiconductor thin film may be such that an electron beamdiffraction pattern of the semiconductor thin film has particularregularity due to {110} orientation, that each of diffraction spots ofthe electron beam diffraction pattern has a spread that is on a circlehaving its center at the central point of an electron beam irradiationarea, and that a tangential line to each of the diffraction spot fromthe central point of the electron beam irradiation area and a linesegment connecting the central point of the electron beam irradiationarea and the central point of the diffraction spot form an angle that iswithin ±1.5°.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are photographs showing electron beam diffractionpatterns of semiconductor thin films;

FIGS. 2A and 2B schematically show the electron beam diffraction patternof FIG. 1A;

FIGS. 3A-3C illustrate a directional relationship of a semiconductorthin film;

FIGS. 4A-4C illustrate forms of grain boundaries;

FIGS. 5A-5E and 6A-6D show a manufacturing process of a semiconductordevice according to a first embodiment of the present invention;

FIG. 7 is a sectional view of an electro-optical device according to afifth embodiment of the invention;

FIG. 8 shows an appearance of an active matrix substrate according tothe fifth embodiment;

FIG. 9 is a sectional view of an active matrix substrate according to asixth embodiment of the invention;

FIG. 10 is a sectional view of an active matrix substrate according to aseventh embodiment of the invention;

FIGS. 11A and 11B show examples of three-dimensional semiconductorcircuits according to a ninth embodiment of the invention;

FIGS. 12A-12F show examples of electronic apparatuses according to an11th embodiment of the invention;

FIG. 13 is a graph showing a SIMS measurement result;

FIGS. 14A and 14B are TEM photographs showing crystal grains of crystalsilicon films;

FIGS. 15A-15C illustrate a model for generation and extinction ofdefects;

FIGS. 16A-16C show a manufacturing process of a semiconductor deviceaccording to an eighth embodiment of the present invention;

FIGS. 17A and 17B are TEM photographs showing crystal grains ofsemiconductor thin film;

FIGS. 18A and 18B are TEM photographs showing dark field image of thesemiconductor thin film;

FIG. 19 is a TEM photograph showing light field image of thesemiconductor thin film;

FIGS. 20A-20C are TEM photographs showing a state of grain boundaries ofthe semiconductor thin film;

FIGS. 21A-21C are TEM photographs showing a state of grain boundaries ofthe semiconductor thin film;

FIGS. 22A-22C are TEM photographs showing a state of grain boundaries ofthe semiconductor thin film;

FIGS. 23A and 23B are views showing a top and a sectional structure of apixel TFT, respectively; and

FIG. 24 is a TEM photograph showing a sectional structure of a pixelTFT.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention as summarized above will be hereinafter describedin detail by using embodiments.

Embodiment 1

This embodiment is directed to a manufacturing process of asemiconductor thin film according to the invention and a semiconductordevice (specifically, a TFT) using the semiconductor thin film as itsactive layer. The manufacturing process will be described basically withreference to FIGS. 5A-5E to 6A-6D.

First, a silicon substrate 501 as a substrate having an insulatingsurface is prepared. The silicon substrate 501 has been deoxidized by ahydrogen heat treatment. A thermal oxidation film 502 is then formed byperforming thermal oxidation on the silicon substrate 501 in anatmosphere containing a halide gas (in this embodiment, a HCl gas).

The resulting thermal oxidation film 502 has a feature that it is muchsuperior in flatness. In this embodiment, by optimizing the thermaloxidation conditions, a thermal oxidation film can be obtained in whichthe average of height differences between recesses and protrusions is 5nm or less (typically 3 nm or less; preferably 2 nm or less) or, if 100recess/protrusion pairs are examined, the recess/protrusion heightdifference is 10 nm or less for all of the 100 pairs and is 5 nm or lessfor 90 of those 100 pairs.

In this manner, a substrate having an extremely flat insulating surfaceis obtained as shown in FIG. 5A. The superior flatness plays animportant role in forming a semiconductor thin film according to theinvention.

Then, an amorphous silicon film 503 is formed at such a thickness thatthe final thickness (i.e., a thickness after thickness reduction due tothermal oxidation) will become 10-75 nm (preferably 15-45 nm). In thisembodiment, the film formation is performed by low-pressure CVD underthe following conditions:

-   -   Film forming temperature: 465° C.    -   Film forming pressure: 0.5 Torr    -   Film forming gases:        -   He (helium) 300 sccm        -   Si₂H₆ (disilane) 250 sccm

It is important that the concentrations of impurities in the film bethoroughly managed during the film formation. In this embodiment, theconcentrations of C (carbon), N (nitrogen), and O (oxygen) that areimpurities impairing the crystallization of the amorphous silicon film503 when existing therein are managed as follows. The concentration ofeach of C and N is controlled so as to be less than 5×10¹⁸ atoms/cm³(typically 5×10¹⁷ atoms/cm³ or less; preferably 2×10¹⁷ atoms/cm³ orless). The concentration of O is controlled so as to be less than1.5×10¹⁹ atoms/cm³ (typically 1×10¹⁸ atoms/cm³ or less; preferably5×10¹⁷ atoms/cm³ or less). This is because if any of these impuritiesexists at a concentration higher than the above value, it will adverselyaffect the crystallization that will be performed later, possiblylowering the film quality after the crystallization.

FIG. 13 shows a result of a SIMS (secondary ion mass spectroscopy)analysis in which the concentrations of impurities in an amorphoussilicon film formed under the conditions of this embodiment weremeasured. The sample was such that a 0.5-μm-thick amorphous silicon filmwas formed on a silicon wafer. As seen from FIG. 13, it was confirmedthat the concentrations of all of C, N, and O fell within the aboveranges. It is noted that in this specification the concentration of anelement in a film is defined by a minimum value in a SIMS measurementresult.

To provide the above features, it is desirable that a low-pressure CVDfurnace used in this embodiment be subjected to dry cleaning on aregular basis to keep its film forming chamber clean.

The dry cleaning may be performed by causing a CIF₃ (chlorine fluoride)gas to flow at 100-300 sccm through the furnace that is heated to about200°-400° C. and cleaning the film forming chamber by fluorine that isgenerated by thermal decomposition.

According to the knowledge of the inventors, deposits (mostly made ofmaterials having silicon as the main component) of about 2 μm inthickness can be removed completely in 4 hours when the intrafurancetemperature is set at 300° C. and the flow rate of a CIF₃ gas is set at300 sccm.

The concentration of hydrogen in the amorphous silicon film 503 is alsoan important parameter; a film of better crystallinity appears to beobtained by making the hydrogen content smaller. Therefore, it ispreferable that the amorphous silicon film 503 be formed by low-pressureCVD. It is possible to use plasma CVD by optimizing the film formingconditions.

Then, a step of crystallizing the amorphous silicon film 503 is executedby using the technique developed by the inventors that is disclosed inJapanese Laid-open Patent Publication No. Hei. 7-130652.

Although either of techniques described in the first and secondembodiments of this publication may be used, as far as this invention isconcerned use of the technique of the second embodiment (described indetail in Japanese Laid-open Patent Publication No. Hei. 8-78329) ispreferable. The disclosures of which are incorporated herein byreference.

According to the technique described in the publication No. Hei.8-78329, first a mask insulating film 504 for selecting a catalystelement adding region is formed. Then, a Ni containing layer 505 isformed by applying, by spin coating, a solution containing nickel (Ni)as a catalyst element for accelerating crystallization of the amorphoussilicon film 503 (see FIG. 5B).

Examples of usable catalyst elements other than nickel are cobalt (Co),iron (Fe), palladium (Pd), platinum (Pt), copper (Cu), gold (Au),germanium (Ge), lead (Pb), and indium (In).

The method of adding the catalyst element is not limited to spincoating, and ion implantation or plasma doping each of which uses aresist mask may also be employed. The latter case is effective inconstructing more miniaturized circuits because the catalyst elementadding region is small and the growth length of a lateral growth regioncan be controlled easily.

After the catalyst element adding step has finished, hydrogen removal isperformed at 450° C. for 1 hour and then the amorphous silicon film 503is crystallized by performing a heat treatment at 500°-700° C.(typically 550°-650° C.) for 4-24 hours in an inert gas, hydrogen, oroxygen atmosphere. In this embodiment, a heat treatment is performed at570° C. for 14 hours in a nitrogen atmosphere.

At this time, the crystallization of the amorphous silicon film 503proceeds with priority from nuclei occurring in a nickel-added region506 and a crystal region 507 is formed as a result of growth that isparallel with the surface of the substrate 501 (see FIG. 5C). Theinventors call the crystal region 507 a lateral growth region. Thelateral growth region is advantageous in having superior crystallinityas a whole because it is a collection of crystals having generally equalcrystallinity.

After the heat treatment for crystallization has finished, the maskinsulating film 504 is removed and then patterning is performed to forman island-like semiconductor layer (active layer) 508 all of which isconstituted of part of the lateral growth region 507. Then, a gateinsulating film 509 is formed that is an insulating film containingsilicon. The thickness of the gate insulating film 509 may be adjustedin a range of 20-250 nm in terms of the thickness after being increasedin a later thermal oxidation step. The film forming method may be aknown vapor-phase method (plasma CVD, sputtering, or the like).

Then, as shown in FIG. 5D, a heat treatment (catalyst element getteringprocess) is performed to remove or reduce the concentration of thecatalyst element (nickel). In this heat treatment, a halogen element isincluded in the processing atmosphere to utilize the metal elementgettering effect of the halogen element.

To fully effectuate the gettering effect of the halogen element, it ispreferable to perform the heat treatment at a temperature higher than700° C. At 700° C. or less, there is a possibility that the halogencompound in the processing atmosphere are hardly decomposed and thegettering effect is not obtained. In view of this, the heat treatmenttemperature is preferably set at 800°-1,000° C. (typically 950° C.) andthe processing time is set at 0.1-6 hours (typically 0.5-1 hour).

A typical example is such that a heat treatment is performed at 950° C.for 30 minutes in an atmosphere containing hydrogen chloride (HCl) at0.5-10 vol % (in this embodiment, 3 vol %) with respect to oxygen. A HCldensity higher than the above range is not preferable because asperitieswhose depth or height is equivalent to the thickness of the active layer508 are formed.

Other than a HCl gas, one or plural kinds of compounds selected fromcompounds including a halogen element of HF, NF₃, HBr, Cl₂, CIF₃, BCl₃,F₂, Br₂, etc. may also be used.

In this step, nickel in the active layer 508 is gettered through theaction of chlorine and removed into the air in the form of nickelchloride that is volatile. After the execution of this step, theconcentration of nickel in the active layer 508 is decreased to 5×10¹⁷atoms/cm³ or less (typically 2×10¹⁷ atoms/cm³ or less). According to theexperience of the inventors, a nickel concentration of 1×10¹⁸ atoms/cm³or less (preferably 5×10¹⁷ atoms/cm³ or less) does not cause any adverseeffects on the TFT characteristics.

The above gettering treatment is also effective for metal elements otherthan nickel. Metal elements that may be mixed into the silicon film aremainly the constituent elements (typically aluminum, iron, chromium,etc.) of the film forming chamber. The above gettering treatment canreduce the concentrations of these metal elements to 5×10¹⁷ atoms/cm³ orless (preferably 2×10¹⁷ atoms/cm³ or less).

After the above gettering, treatment, the halogen element that was usedin the gettering treatment remains in the active layer 508 at aconcentration of 1×10¹⁵ to 1×10²⁰ atoms/cm³.

In the above heat treatment, thermal oxidation reaction proceeds at theinterface between the active layer 508 and the gate insulating film 509,whereby the thickness of the gate insulating film 509 increases as muchas the thickness of a resulting thermal oxidation film. By forming athermal oxidation film in this manner, a semiconductor/insulating filminterface with a very small number of interface states can be obtained.The heat treatment also provides an effect that a failure in the thermaloxidation film formation at the ends of the active layer 508 (edgethinning) can be prevented.

It is also effective to improve the film quality of the gate insulatingfilm 509 by performing a heat treatment at 950° C. for 1 hour in anitrogen atmosphere after the above heat treatment in a halogenatmosphere.

It is to be noted that a halogen element gettering process may beperformed between a crystallization step and a film formation step for agate insulating film.

Thereafter, a metal film (not shown) having aluminum as the maincomponent is formed and then patterned into a gate electrode startingmember 510 (see FIG. 5E). In this embodiment, an aluminum filmcontaining scandium at 2 wt % is formed. Other than such an aluminumfilm, a tantalum film, a conductive silicon film, etc. may also be used.

At this stage, a technique of the inventors that is disclosed inJapanese Laid-open Patent Publication No. Hei. 7-135318 is used. Thedisclosures of which are incorporated herein by reference. Thispublication discloses a technique of forming source and drain regionsand low-concentration impurity regions in a self-aligned manner by usingan oxide film formed by anodization. This technique will be describedbelow briefly.

First, in a state that a resist film (not shown) that was used forpatterning the aluminum film is left as it is, anodization is performedin a 3%-aqueous solution of oxalic acid, whereby a porous anodic oxidefilm 511 is formed. Since the thickness of the anodic oxide film 511corresponds to the length of low-concentration impurity regions thatwill be formed later, the former is controlled so that the latter willhave a desired value.

Then, after the resist film (not shown) is removed, anodization isperformed in an electrolyte obtained by mixing tartaric acid (3%) intoan ethylene glycol solution. As a result, a dense, non-porous anodicoxide film 512 is formed. Its thickness may be set at 70-120 nm.

An aluminum film 513 that remains after the above two anodization stepswill substantially serve as a gate electrode (see FIG. 6A).

Then, the gate insulating film 509 is etched by dry etching by using thegate electrode 513 and the porous anodic oxide film 511 as a mask. Theporous anodic oxide film 511 is then removed. The end portions of aresulting gate insulating film 514 project by the thickness of theporous anodic oxide film 511 (see FIG. 6B).

Then, a step of adding an impurity element for imparting oneconductivity type is performed. The impurity element may be P(phosphorus) or As (arsenic) for n-type conductivity and B (boron) or In(indium) for p-type conductivity.

In this step, first impurity addition is performed at a highacceleration voltage to form n− regions. Because the accelerationvoltage is as high as about 80 kV, the impurity element is added to notonly the exposed portions of the active layer 508 but also the portionsof the active layer 508 located under the exposed end portions of thegate insulating film 514. Second impurity addition is then performed ata low acceleration voltage to form n+ regions. Because the accelerationvoltage is as low as 10 kV, the gate insulating film 514 serves as amask.

Among the impurity regions formed by the above step, the n+ regionsbecome a source region 515 and a drain region 516 and the n− regionsbecome a pair of low-concentration impurity regions (one of which iscalled a LDD region) 517. The impurity element is not added to theportion of the active layer 508 right under the gate electrode 513,which becomes an intrinsic or substantially intrinsic channel formingregion 518 (see FIG. 6C).

After the active layer has been completed in the above manner, theimpurity element is activated by a combination of furnace annealing,laser annealing, lamp annealing, etc. At the same time, damage of theactive layer caused in the impurity element adding step is repaired.

Then, a 500-nm-thick interlayer insulating film 519 is formed, which maybe a silicon oxide film, a silicon nitride film, a silicon oxynitridefilm, an organic resin film, or a multilayered film thereof.

Then, after contact holes are formed, a source electrode 520 and a drainelectrode 521 are formed. Finally, the entire device is hydrogenated byheating the entire substrate at 350° C. for 1-2 hours, whereby danglingbonds in the films (particularly in the active layer) are terminated. ATFT having a structure shown in FIG. 6D is thus manufactured.

Since the invention is directed to the technique relating to asemiconductor thin film that constitutes an active layer, the otherstructures and configurations do not restrict the invention at all.Therefore, the invention can easily be applied to TFTs having differentstructures and configurations than in this embodiment.

Knowledge Relating to Impurities Contained in Active Layer

An active layer (semiconductor thin film) of this embodiment has afeature that it contains no or substantially no C (carbon), N(nitrogen), and O (oxygen), which are elements imparting thecrystallization of the active layer. This feature is obtained bythorough management of impurities (pollutants).

In this embodiment, the mixing of C, N, and O is thoroughly avoided informing an amorphous silicon film, necessarily resulting in the featurethat the concentration of each of C and N in a final semiconductor filmis at most less than 5×10¹⁸ atoms/cm³ (typically 5×10¹⁷ atoms/cm³ orless; preferably 2×10¹⁷ atoms/cm³ or less) and the concentration of O isat most less than 1.5×10¹⁹ atoms/cm³ (typically 1×10¹⁸ atoms/cm³ orless; preferably 5×10¹⁷ atoms/cm³ or less).

A pure semiconductor film that is made of only silicon has a siliconconcentration of about 5×10²² atoms/cm³. Therefore, for example, animpurity element concentration of 5×10¹⁸ atoms/cm³ corresponds to about0.01 atomic %.

To obtain superior crystallinity, it is desirable that theconcentrations of C, N, and O in the final semiconductor film be lessthan the detection limit of a SIMS analysis and it is more desirablethat the final semiconductor film contain no such impurities at all.

SIMS analyses of the inventors revealed that if an amorphous siliconfilm in which the concentrations of C, N, and O fall within the aboveranges is used as a starting film, the concentrations of C, N, and Ocontained in the active layer of a completed TFT also fall within theabove ranges.

Knowledge Relating to Crystal Structure of Active Layer

Microscopically, an active layer formed by the above manufacturingprocess has a crystal structure in which a plurality of rod-like (orflat-rod-like) crystals are arranged approximately parallel with eachother with such regularity that they are directed to a particulardirection. This can easily be confirmed by an observation by a TEM(transmission electron microscope) method.

FIGS. 17A and 17B are HR-TEM photographs with 8 million times ofmagnification of the grain boundaries comprising rod-like orflat-rod-like crystals. In this specification, the grain boundary isdefined, unless otherwise specified, as one that is formed at theinterface where the rod-like crystals or the flat-rod-like crystals arecontacted with each other. Accordingly, it is distinguished from amacroscopic grain boundary that is formed by, for instance, collision oflateral growth regions.

The above-mentioned HR-TEM (high-resolution transmission electronmicroscope) method is a technique in which an electron beam isvertically applied to a sample and an arrangement of atoms or moleculesis evaluated by utilizing interference among transmitted electrons orelastically scattered electrons.

By using HR-TEM, an arrangement state of crystal lattices can beobserved as a lattice fringe. Therefore, by observing grain boundaries,a bonding state of atoms at grain boundaries can be estimated.Incidentally, the lattice fringe appears as a fringe consisting of whiteand black. However, it occurs due to a difference of contrast, and doesnot indicate the positions of atoms.

FIG. 17A is a typical TEM photograph of a crystalline silicon film thatis obtained according to the present invention. A state that twodifferent crystal grains contact each other at the grain boundariesshown from the upper left to the right lower was observed. In this case,the two crystal grains approximately had {110} orientation though therewas a small shift between their crystal axes.

It is confirmed by x-ray diffraction or electron beam diffraction that,as a result of checking a plurality of crystal grains, the crystalgrains approximately had {110} orientation in most of cases, which willbe described later. Incidentally, among the results of the observationsin many cases, a (011) plane or a (200) plane may be found, however,those having an equivalent value is expressed totally as a {111} plane.

As shown in FIG. 17A, a lattice fringe corresponding to a {111} plane ora {100} plane was found in a lattice fringe of the {110} plane. The“lattice fringe corresponding to the {111} plane” means a such latticefringe that a {111} plane appears as a cross-section obtained by cuttinga crystal grain along the lattice fringe. What plane a lattice fringecorresponds to can be checked with simply based on intervals of thelattice fringes.

In FIG. 17A, there is a difference in the visible states of the latticefringes, however, which causes a delicate difference in inclinations ofthe crystal grains. That is, when it is set so that electron beam isvertically irradiated onto one crystal plane of the crystal grains,since the other crystal grains become a state that the electron beam isactually irradiated with an inclination thereonto, the views of thelattice fringes are changed.

Here, attention is paid on the lattice fringe corresponding to the {111}plane. In FIG. 17A, the lattice fringe of the crystal grainscorresponding to the {111} plane, located upper side while a crossingthe grain boundaries, intersects the lattice fringe of the crystalgrains corresponding to the {111} plane, located lower side, at about70° (more correctly 70.5°).

This indicates a crystal structure (more correctly, a grain boundarystructure) that two different crystal grains are connected to each otherat the grain boundaries with an extremely high degree of matching. Thatis, crystal lattices are continuously connected to each other at grainboundaries and trap states due to crystal defects etc. are far lessprone to occur. In other words, crystal lattices have continuity atgrain boundaries.

For the reference, an HR-TEM photograph of conventional high-temperaturepolysilicon film is shown in FIG. 17B. In case of FIG. 17B, crystalplane has no regularity, to be described later, the {110} plane does notconstitute main orientation. However, in FIG. 17B, in order to comparewith FIG. 17A, crystal grains in which the lattice fringe correspondingto the {111} plane appears were observed.

Observing FIG. 17B in detail, as shown in the figure by an arrow, it wasfound many portion where the lattice fringes are disconnected at thegrain boundaries. In such portions, dangling bond (called crystaldefect) exists, and there is a risk that such defect inhibits movementof carriers as a trap level.

Actually, dangling bond as shown in FIG. 17B exists in the crystallinesilicon film according to the present invention. This result is notavoidable as long as a crystalline silicon film according to the presentinvention is a polycrystal. However, as a result of observing thecrystalline silicon film of the present invention extendedly in detailby TEM, it was found that extremely few dangling bonds existed therein.

As long as the present inventors carried out observation, it was foundthat crystal lattices were continuous in 90% or more (typically 95% ormore), as a whole, of grain boundaries, and few dangling bonds as shownin FIG. 17B were observed. From this result, it can also say that thecrystalline silicon film according to the present invention clearlydiffers from the conventional high temperature polysilicon.

Then, FIG. 1A shows a result of an electron beam diffraction analysis ona semiconductor thin film according to the invention. FIG. 1B shows anelectron beam diffraction pattern of a conventional polysilicon film asa reference. In FIGS. 1A and 1B, the diameters of electron beamapplication areas are 4.25 μm and 1.35 μm, respectively. Thesephotographs are typical ones selected from photographs taken at aplurality of locations.

In the case of FIG. 1A, diffraction spots corresponding to <110>incidence appear relatively clearly and it is confirmed that almost allcrystal grains in the electron beam application area have {110}orientation.

Incidentally, the present inventors carried out x-ray diffraction inaccordance with a technique disclosed in Japanese Laid-open PatentPublication No. Hei. 7-321339, and an orientation ratio of thesemiconductor thin film according to the present invention wascalculated. The disclosure of which is incorporated herein by reference.In the patent publication, the orientation ratio was calculated inaccordance with the calculation method as shown in Equation 1.{220} orientation abundance=1 (fixed){111} orientation abundance=relative strength of sample {111} to{220}/relative strength of powder {111} to {220}{311} orientation abundance=relative strength of sample {311} to{220}/relative strength of powder {311} to {220}{220} orientation ratio {220} orientation abundance/{220} orientationabundance+{111} orientation abundance+{311} orientationabundance  [Equation 1]

As a result of checking by x-ray diffraction an orientation ofsemiconductor thin film according to the present invention, a peakcorresponding to a (220) plane was appeared in an x-ray diffractionpattern. It is needless to say that the (220) plane has an equivalentvalue to the {110} plane. As a result, it was confirmed that the {110}plane was a main orientation plane, and the orientation ratio was 0.7 ormore (typically 0.9 or more).

On the other hand, in the case of the conventional high-temperaturepolysilicon film shown in FIG. 1B, diffraction spots do not have clearregularity and are oriented approximately randomly; in other words, itis confirmed that crystal grains having various kinds of planeorientation other than the {110} orientation are mixed in an irregularmanner.

Incidentally, each diffraction spot has slight spreads on concentriccircles, which is, however, considered due to a certain distribution inthe rotation angle around the crystal axis. This will be describedbelow.

FIG. 2A schematically shows part of the electron beam diffractionpattern of FIG. 1A. In FIG. 2A, a plurality of bright spots 201, whichare diffraction spots corresponding to the <110> incidence, aredistributed in a concentric manner with a central point 202 of theirradiation area as the center.

FIG. 2B is an enlarged version of a region 203 surrounded by a brokenline in FIG. 2A. As shown in FIG. 2B, a detailed examination of theelectron beam diffraction pattern of FIG. 1A shows that a diffractionspot 201 has a spread (fluctuation) of about ±1.5° with respect to thecentral point 202 of the irradiation area.

That is, the angle formed by a tangential line 204 to the diffractionspot 201 from the central point 202 of the electron beam irradiationarea and a line segment connecting the central point 202 of the electronbeam irradiation area and a central point 205 of the diffraction spot(this angle corresponds to ½ of the rotation angle) is 1.5° or less.Since two tangential lines can be drawn, the spread of the diffractionspot 201 is in the range of ±1.5°.

This tendency is found in the entire area of the electron beamdiffraction pattern of FIG. 1A and the spreads of the diffraction spotsfall within ±2.5° (typically within ±1.5°; preferably within 0.5°). Theabove-mentioned sentence “each diffraction spot has slight spreads onconcentric circles” means this tendency.

The ratio (a/b) of the length (a) of the minor axis of the diffractionspot 201 to its major-axis length (b) can be made equal to 1/1 (circle)to 1/1.5 by making the underlying surface of a semiconductor thin filmas close to a complete flat surface as possible. This means thatdiffraction spots become circular or substantially circular.

To make diffraction spots circular, the rotation angle among a pluralityof crystal grains should be made very small. Diffraction spots of anelectron beam diffraction pattern of a single crystal are completelycircular. Therefore, making diffraction spots circular means making asemiconductor thin film of the invention as close to a single crystal aspossible.

FIG. 3A shows a relationship between the crystal axis and axes includedin a crystal surface in a case where the plane orientation is {110}. Asshown in FIG. 3A, in the case of a crystal surface having {110}orientation, the crystal axis is the <110> axis and the <111> axis, the<100> axis, etc. exist in the crystal surface.

The inventors previously studied the growth direction of rod-likecrystals of the above-mentioned kind by the HR-TEM method, and it wasconfirmed that they grew approximately along the <111> axis (refer toJapanese Laid-open Patent Publication No. Hei. 7-321339). Thedisclosures of which are incorporated herein by reference. Therefore, itis considered that part of a semiconductor thin film of the invention isas shown in an enlarged view of FIG. 3B.

In FIG. 3B, reference numerals 301-303 denote different rod-likecrystals and the crystal axes of the respective crystal grainsapproximately coincide with the <110> axis. Since on average the crystalgrowth proceeds approximately along the <111> axis, the rod-likecrystals extending directions approximately coincide with the <111> axisdirections. Broken lines indicate grain boundaries.

In this case, if a <111> axis 304 included in the surface of anarbitrary crystal grain 301 is employed as a reference axis, <111> axes305 and 306 included in the surfaces of other nearby rod-like crystals302 and 303 coincide with the reference axis 304 or slightly deviatefrom the reference axis 304 and form certain angles with it,respectively. In this specification, this angle is called the “rotationangle.”

The above-mentioned fact that the spreads of respective diffractionspots fall within ±2.5° (typically within ±1.5°; preferably within±0.5°) has the same meaning as that the absolute values of respectiverotation angles are within 5° (typically within 3°; preferably within1°).

FIG. 3C summarizes the above relationship. In a semiconductor thin filmof the invention, an angle (α) formed by the axis 305 and the referenceaxis 304, and an angle (β) formed by the axis 306 and the reference axis304 are rotation angles and they are at most within 5°.

Crystal grains having slightly different rotation angles as shown inFIG. 3B appear as different diffraction spots in an electron beamdiffraction pattern. For example, diffraction spots of the crystalgrains 302 and 303 deviate from a diffraction spot of the crystal grain301 by the rotation angles α and β, respectively, on a concentriccircle.

That is, if a plurality of crystal grains exist in a electron beamirradiation area, diffraction spots corresponding to those plurality ofcrystal grains are arranged continuously on a concentric circle, wherebya resulting diffraction spot has an apparent shape that is close to anellipse. This is the reason why each diffraction spot of the electronbeam diffraction pattern of FIG. 1A has a spread.

The notation of, for instance, <111> used in this specification includesequivalent axes [111], [1−11], etc. (the minus sign means inversion).Diffraction spots appear for all of the equivalent axes so as to form anelectron beam diffraction pattern as shown in FIG. 1A. If crystal grainsare rotated by a certain rotation angle, an electron beam diffractionpattern is also rotated as a whole by the rotation angle. Therefore, alldiffraction spots have a spread on a concentric circle.

It is concluded that the reason why a diffraction pattern as shown inFIG. 1A was obtained when a semiconductor thin film of the invention wasexamined by electron beam diffraction is that a plurality of rod-likecrystals existed in the electron beam irradiation area and had slightlydifferent rotation angles. Based on the spreads of respectivediffraction spots, the absolute values of rotation angles are estimatedto be within 5° (typically within 3°; preferably within 1°)

This means that a deviation between arbitrary reference axes of twocrystal grains having the largest rotation angles among all crystalgrains constituting a semiconductor thin film of the invention is within5°.

Now, a description will be made of the degree of existence of variouskinds of crystal grains in a semiconductor thin film of the inventionaccording to the common grain boundary classification. Table 1 has beenobtained based on data of semiconductor thin films of the invention.

TABLE 1 Semiconductor thin Kinds of grain boundaries Features film ofinvention Remarks Small-angle boundary Small inclination angle Slightlyrotated None or If these kinds of boundaries (rotational relationshipboundary about direction substantially do not exist or exist at a ofless than approx. 15°) included in non-existent very small percentage,the boundary surface crystal can be regarded (Small) twisted boundarySlightly rotated (substantially) as a single about direction crystal.perpendicular to boundary surface. Special large-angle Twin boundaryRotated by 180° Small percentage boundary about certain (Hard toeliminate common direction. it completely because it is highly stable interms of energy among various defects.) Other correspondence Latticepoints None or boundaries common to grains substantially on both sidesof non-existent boundary exist at certain percentage. Random large-angleboundary No meaningful None or If boundaries of this kind directionalsubstantially exist, the crystal cannot relationship. non-existent beregarded even as a quasi-single crystal.

The several kinds of grain boundaries shown in Table 1 can bediscriminated from each other by making good use of electron beamdiffraction, an HR-TEM method, a cross-sectional TEM method, etc., andeven more detailed information can be obtained. Values of the rotationangle appearing in this specification are ones measured by analyzinggrain boundaries from various aspects by combining the above techniques.

Grain boundaries of the above-mentioned rotation about a crystal axisare classified as the small inclination angle boundary because it is a“rotation about a direction included in the boundary surface.” Informing this type of grain boundary, two crystal grains contact eachother in a relationship schematically shown in FIG. 4A. In this case,the surface where the two crystal grains contact each other is aboundary surface. However, in a semiconductor thin film of theinvention, it is possible to consider that grain boundaries of the kindshown in FIG. 4A do not exist because the rotation angle about thecrystal axis is as extremely small as within ±2.5°.

The small inclination angle boundary includes a version shown in FIG.4B. In this case, the rotation axis is different than in FIG. 4A.However, the grain boundary of FIG. 4B is the same as that of FIG. 4A inthat two crystal grains form a certain rotation angle about an axisincluded in the boundary surface. In a semiconductor thin film of theinvention, it is also possible to consider that grain boundaries of thiskind do not exist because the rotation angle is within ±2.50 (typicallywithin ±1.5°; preferably within ±0.5°).

The small-angle boundary also includes a form called the twistedboundary which is distinguished from the small inclination angleboundary shown in FIGS. 4A and 4B. As shown in FIG. 4C, the twistedboundary corresponds to a case where the rotation is about an axis thatis perpendicular to the boundary surface.

The twisted boundary is the same as the small inclination angle boundaryin that two crystal grains form a certain rotation angle. In asemiconductor thin film of the invention, the rotation angle is within±2.5° (typically within ±1.5°; preferably within ±0.5°). That is, it ispossible to consider that there are almost no twisted boundaries.

As described above, it is possible to consider that a semiconductor thinfilm of the invention has no or substantially no electrically activegrain boundaries commonly called the small-angle boundary. It is to benoted that the term “electrically active” means that a carrier canfunction as a trap.

Also, the term “substantially no” means that at most one or two grainboundaries of the kind concerned (for instance, the small-angleboundary) are found when grain boundaries in an area of, for instance, 5μm² are examined.

The special large-angle boundary includes the twin boundary and theother correspondence boundaries. However, it is confirmed that almost ofthe semiconductor thin films of the invention are twin boundaries. Also,it was confirmed that even if the correspondence boundaries exist, theyare electrically inactive (not function as traps).

In the semiconductor thin film of the invention, in special, thecorrespondence boundaries ({111} twin boundaries) of 3 make up 90%(typically 95% or more) of entire grain boundaries, and therefore it isextensively proved that the grain boundaries with high degree ofmatching are formed therein.

The value of Σ is a parameter as a guideline for indicating the degreeof matching of the correspondence boundaries, and it is known that asthe value of Σ becomes lower, the higher the degree of matching of thegrain boundaries becomes. As to the definition of Σ value, it isdescribed in detail in “High resolving power electron microscopy forevaluating material,” written jointly by Daisuke Shindo and KenjiHiraga, pp. 54-60, Kyoritsu Shuppan K. K 1996. The disclosure of whichis incorporated herein by reference.

In the grain boundaries formed between two crystal grains, in the casewhere the plane orientations of both crystals are {110}, if to an angleformed by lattice fringes corresponding to a {111} plane is defined asθ, it is known that the grain boundaries become the correspondenceboundaries of Σ3 when θ=70.5°.

Therefore, in the grain boundaries shown in a TEM photograph of FIG. 1A,each lattice fringe of adjacent crystal grains is continuous with anangle of 70.5°, thereby being capable of easily inferring that the grainboundaries are twin boundaries of {111}.

It is to be noted that when θ=38.9°, the grain boundaries become thecorrespondence boundaries of Σ9. However, such other correspondenceboundaries slightly existed therein.

The above-mentioned correspondence boundaries are formed only betweengrain boundaries oriented in the same direction. That is, the planeorientation of the semiconductor thin film of the inventionsubstantially aligned with {110}. As a result, it is possible to formthe correspondence boundaries covering a wide area. This feature doesnot appear in other polysilicon film with an irregular planeorientation.

Further, the random large-angle boundary is a grain boundary that isfound in a semiconductor film in which crystal grains are arranged inirregular directions, that is, without any meaningful directionalrelationship. Such a conventional semiconductor thin film as ahigh-temperature polysilicon film has many grain boundaries of thiskind. Naturally, a semiconductor thin film of the invention has almostno random large-angle boundaries.

If neither the small-angle boundary nor the special random large-angleboundary (see Table 1) exists or they exist in an extremely smallnumber, it is possible to consider that no grain boundaries exist. Thatis, a semiconductor thin film having such a crystal structure can beconsidered a single crystal or so considered substantially, having nosubstantial grain boundaries.

In a semiconductor thin film of the invention, the main orientationplane is the {110} plane over the entire film because each crystal grainis approximately {110}-oriented. Although individual crystal grains formcertain rotation angles with each other, the rotation angles are within±2.5° (typically within ±1.5°; preferably within ±0.5°). Therefore, itcan be considered that substantially no grain boundaries exist. Theabove discussions lead to a conclusion that a semiconductor thin film ofthe invention can be considered a single crystal or so consideredsubstantially.

As described above, in a semiconductor thin film of the invention,individual crystal grains constituting the thin film are oriented in thesame direction or in a relationship having a certain rotation angle. Therotation angles are as very small as within ±2.5°, which is at such alevel as to be regarded as not forming grain boundaries substantially.

The inventors attach importance to the flatness of the underlyingsurface as a reason why such a semiconductor thin is obtained. Accordingto the experiences of the inventors, irregularities on the underlyingsurface greatly affect the crystal growth. That is, irregularities etc.on the underlying surface cause strains or the like in crystal grains,to cause deviations or the like of the crystal axis.

A semiconductor thin film of the invention is formed on an undercoatfilm very high in flatness that is formed by the method as described inthis embodiment. Since crystal growth proceeds in a state that thefactors impairing the crystal growth are eliminated as much as possible,crystal grains join each other while very high crystallinity ismaintained. It is considered that a semiconductor thin film having suchcrystallinity as can substantially be regarded as a single crystal isobtained as a result of the above crystal growth.

In forming a semiconductor thin film of the invention, the annealingstep (in this embodiment, the step of FIG. 5D) that is performed at atemperature higher than the crystallization temperature plays animportant role in decreasing defects in crystal grains. This will beexplained below.

FIG. 14A is a TEM photograph with 250 thousand times of magnification ofa crystal silicon film at the time point when the crystallization stepof FIG. 5C has finished. A zigzagged defect (indicated by an arrow) isfound in crystal grains (black and white portions appear due to adifference in contrast).

Although the defect of this type is mainly a stacking fault caused byerroneous stacking order of atoms in silicon lattice planes, it may be adislocation or some other defect. The defect of FIG. 14A appears to be astacking fault having a defect surface parallel with the {111} plane, asjudged from the fact that the zigzagged defect is bent at about 70°.

On the other hand, as shown in FIG. 14B, a crystal silicon film of theinvention that is viewed with the same magnification has almost nodefects of stacking faults, dislocations, etc. in crystal grains andhence has very high crystallinity. This tendency holds over the entirefilm surface. Although it is currently difficult to make the number ofdefects zero, it is possible to decrease it to such a level as cansubstantially be regarded as zero.

That is, the crystal silicon film of FIG. 14B can be considered a singlecrystal or so considered substantially because the number of defects incrystal grains is reduced to an almost negligible level and grainboundaries never become barriers for carrier movement by virtue of theirhigh continuity.

As described above, although the crystal silicon films shown in thephotographs of FIGS. 14A and 14B are approximately the same incontinuity, they are much different in the number of defects in crystalgrains. The fact that the crystal silicon film of this embodimentexhibits far superior electrical characteristics to the crystal siliconfilm of FIG. 14A is largely due to the difference in the number ofdefects.

The present assignee assumes the following model for phenomena occurringin the step of FIG. 5D. First, in the state of FIG. 14A, atoms of thecatalyst element (typically nickel) are segregated in defects (mainlystacking faults) in crystal grains. That is, it is considered that thereexist many Si—Ni—Si type bonds.

When the catalyst element gettering process is executed, Ni atomsexisting in defects are removed and Si—Ni bonds are disconnected. Excessbonds of Si atoms immediately form Si—Si bonds to establish a stablestate. The defects disappear in this manner.

It is known that defects in a crystal silicon film disappear whenhigh-temperature annealing is performed. It is inferred that in theinvention the silicon recombination occurs more smoothly because manydangling bonds are generated by disconnection of Si—Ni bonds.

Further, it is considered that excess silicon atoms that are generatedat the same time when the crystal silicon film is thermally oxidizedmove to defects and greatly contribute to formation of Si—Si bonds. Thisis the same notion as explains why a high-temperature polysilicon filmhas only a small number of defects in crystal grains.

The present assignee also assumes a model that the heat treatment at atemperature (typically 700°-1,100° C.) higher than the crystallizationtemperature causes the crystal silicon film to be fixed to theunderlying surface to improve the adhesion there, which in turn causesdefects to disappear.

The crystal silicon film and the silicon oxide film as the undercoatfilm are different from each other in thermal expansion coefficient by afactor close to 10. Therefore, after the amorphous silicon film has beenconverted into the crystal silicon film (see FIG. 14A), very strongstress is imposed on the crystal silicon film during its cooling.

This will be explained below with reference to FIGS. 15A-15C. FIG. 15Ashows a heat history to which a crystal silicon film is subjected afterthe crystallization step. First, a crystal silicon film that has beencrystallized at temperature t₁ is cooled to the room temperature in acooling period (a).

FIG. 15B shows the crystal silicon film that is in the midst of thecooling period (a). Reference numerals 30 and 31 denote a quartzsubstrate and a crystal silicon film, respectively. At this stage, theadhesion between the crystal silicon film 31 and the quartz substrate 30at an interface 32 is not high, as a result of which many intragraindefects occur.

That is, it is considered that the crystal silicon film 31 that is giventension due to the difference in thermal expansion coefficient is veryprone to move on the quartz substrate 30 and defects 33 such as stackingfaults and dislocations are easily caused by such force as tensilestress.

The crystal silicon film that is obtained in the above manner is in thestate of FIG. 14A. Thereafter, the catalyst element gettering step isexecuted at temperature t₂ as shown in FIG. 15A, whereby the defects inthe crystal silicon film disappear for the above-described reasons.

In this step, it is an important point that in the catalyst elementgettering step, the crystal silicon film is fixed to the quartzsubstrate 30 to improve the adhesion. That is, the gettering step alsoserves as a step of fixing the crystal silicon film to the quartzsubstrate 30 (underlying member).

After the completion of the gettering and fixing step, the crystalsilicon film is cooled to the room temperature in a cooling period (b).In contrast to the case of the cooling period (a) after thecrystallization step, in the cooling step (b) an interface 35 betweenthe quartz substrate 30 and the annealed crystal silicon film 34 is in astate of very high adhesion (see FIG. 15C).

Where the adhesion is so high, the crystal silicon film 34 is completelyfixed to the quartz substrate 30 and hence stress that is imposed on thecrystal silicon film 34 in the cooling period (b) does not causedefects. That is, it can be prevented that defects are generated again.

Although FIG. 15A shows the process in which the temperature is reducedto the room temperature after the crystallization step, the getteringand fixing step may be performed by increasing the temperatureimmediately after the completion of the crystallization step. Such aprocess can also produce a crystal silicon film of the invention.

The crystal silicon film of the invention obtained in the above manner(see FIG. 14B) has a feature that the number of defects in crystalgrains is much smaller than in the crystal silicon film as subjected tothe crystallization step (see FIG. 14A).

In an electron spin resonance (ESR) analysis, the difference in thenumber of defects appears as a difference in spin density. At present,it has become apparent that the spin density of a crystal to siliconfilm of the invention is 5×10¹⁷ spins/cm³ or less (preferably 3 ×10¹⁷spins/cm³ or less). However, since this measurement value is close tothe detection limit of the currently available measuring instruments, itis considered that the actual spin density is even lower.

The crystal silicon film of the invention having the above crystalstructure and features is called by the present inventors a continuousgrain silicon (CGS) film.

Knowledge Relating to Correspondence Grain Boundary

The correspondence boundaries described above are formed between grainboundaries oriented in the same direction. That is, the planeorientation of the semiconductor thin film of the inventionsubstantially aligned with {110}. As a result, it is possible to formthe correspondence boundaries covering a wide area. This feature doesnot appear in other polysilicon film with an irregular planeorientation.

In this embodiment, a TEM photograph (dark field image) with 15 thousandtimes of magnification of the semiconductor thin film of the inventionis shown in FIG. 18A. There are regions that appear in white or black.However, the portion appeared in the same color indicates the portionhaving the same orientation.

In FIG. 18A, it should be featured that the regions appeared in whiteare considerably continuously collected in such a wide dark field view.This means that the crystal grains having the same orientation existtherein with having a certain direction, and adjacent crystal grainshave substantially the same orientation.

On the other hands, a TEM photograph (dark field image) with 15 thousandtimes of magnification of the conventional high temperature polysiliconfilm is shown in FIG. 18B. In the conventional high temperaturepolysilicon film, the regions having the same plane orientation are onlydispersed therein, and any lump having a certain direction as shown inFIG. 18A cannot be found. It can be considered that the reason thereofis due to the irregularity of the orientation of the adjacent crystalgrains to each other.

FIG. 19 also shows a TEM photograph of the semiconductor thin film ofthe invention which was carried out a nickel element gettering treatmentby a technique described in embodiment 4 of the invention later, and wasobserved in light field. Further, a photograph in which Point 1 in FIG.19 is magnified to 300 thousand times is shown in FIG. 20A. A photographwith 2 million times magnification thereof is also shown in FIG. 20B. Itis to be noted that a region surrounded by a square in FIG. 20Acorresponds to FIG. 20B. FIG. 20C also shows an electron deflectionpattern (spot diameter: 1.7 μmφ) in Point 1.

Further, Point 2 and Point 3 were observed under quite the sameconditions as Point 1. Observation results of Point 2 were shown inFIGS. 21A to 21C, and observation results of Point 3 were shown in FIGS.22A to 22C.

From these observation results, it was confirmed that the continuity ofcrystal lattices were kept so that plane boundaries are formed in anarbitral grain boundary. It should be noted that the inventors haverepeated observation and measurement over a large number of regionsother than the above-indicated measuring points. As a result, it wasconfirmed that the continuity of the crystal lattices in grainboundaries was ensured in a region that is sufficiently wide formanufacturing the TFT.

Embodiment 2

The first embodiment is directed to the case where to provide anunderlying surface that is superior in flatness a silicon substrate isthermally oxidized in an atmosphere containing a halide (for instance,HCl). The second embodiment is directed to a case of using another typeof substrate.

In this embodiment, first an inexpensive, low-grade quartz substrate isprepared. Then, the quartz substrate is polished, by, for instance,chemical mechanical polishing (CMP), into an ideal state that theaverage of height differences of recesses/protrusions is within 5 nm(typically within 3 nm; preferably within 2 nm).

In this manner, an insulating substrate having superior flatness can beobtained by polishing an inexpensive quartz substrate. The use of aquartz substrate enables provision of a very dense underlying surface,in which case the interface between the underlying surface and asemiconductor thin film is made highly stable. Having an additionaladvantage that a semiconductor thin film receives almost no influencesof pollutants from the substrate, a quartz substrate is very high inutility value.

Embodiment 3

While the first embodiment is directed to the case of using a siliconfilm as a semiconductor film, it is effective to use a silicon filmcontaining germanium at 1-10%, which is expressed bySi_(x)Ge_(1-x)(0<X<1; preferably 0.05≦x≦0.95).

By using such a compound semiconductor film, the threshold voltage canbe made small when an n-type or p-type TFT is manufactured. Further, thefield-effect mobility can be increased.

Embodiment 4

The first embodiment is directed to the case where a halogen element isused in the step of gettering a catalyst element for acceleratingcrystallization of silicon. In the invention, it is also to possible touse the element of phosphorus in the catalyst element gettering step.

Phosphorus may be used in such a manner that it is added to regionsother than a region to become an active layer and a heat treatment isperformed at 400°-1,050° C. (preferably 600°-750° C.) for 1 minute to 20hours (typically 30 minutes to 3 hours). As a result of the heattreatment, the catalyst element is gettered in the phosphorus-addedregions, whereby the concentration of the catalyst element in an activelayer is reduced to 5×10¹⁷ atoms/cm³ or less.

After the gettering step has finished, an active layer is formed byusing the region other than the phosphorus-added regions. Then, the samesteps as in the first embodiment are executed, to produce asemiconductor device having the same features as in the firstembodiment.

It goes without saying that if a heat treatment is performed in anatmosphere containing a halogen element in forming a thermal oxidationfilm that is to become a gate insulating film, a multiplier effect ofthe gettering effect of phosphorus according to this embodiment and thegettering effect of the halogen element can be obtained.

Embodiment 5

This embodiment is directed to a case of constructing a reflection-typeliquid crystal panel by using semiconductor devices according to thefirst embodiment. FIG. 7 is a sectional view of an active matrix liquidcrystal panel in which a CMOS circuit is formed in an area of a drivercircuit or a logic circuit and a pixel TFT is formed in an area of apixel matrix circuit.

The CMOS circuit is formed by complementarily combining an n-channel TFTand a p-channel TFT. Since the structure and the manufacturing method ofeach TFT constituting the CMOS circuit are the same as in the firstembodiment, descriptions therefor are omitted.

To produce the pixel TFT, it is necessary to further improve a TFT asused to constitute a driver circuit etc. In FIG. 7, a silicon nitridefilm 701 serves as not only a passivation film of the CMOS circuit butalso an insulator for constituting an auxiliary capacitor.

A titanium film 702 is formed on the silicon nitride film 701, and anauxiliary capacitor is formed between the titanium film 702 and a drainelectrode 703. Since the insulator is a silicon nitride film havinglarge relative permittivity, the capacitance can be made large. Since inthe reflection-type panel there is no need for considering the apertureratio, the structure of FIG. 7 causes no problem.

An interlayer insulating film 704 is an organic resin film, which is apolyimide film in this embodiment. It is preferable to secure flatnessof a sufficiently high level by making the interlayer insulating film704 as thick as about 2 μm. As a result, a pixel electrode 705 havingsuperior flatness can be formed.

The pixel electrode 705 is made of aluminum or a material havingaluminum as the main component. It is better to use a material having ashigh reflectance as possible. Further, by securing superior flatness,the loss due to diffused reflection at the pixel electrode surface canbe reduced.

An alignment film 706 is formed on the pixel electrode 705. Thealignment film 706 is given alignment ability by rubbing it. Thedescription made so far is directed to the configuration of a TFTsubstrate (active matrix substrate).

On the other hand, an opposed substrate is constructed by forming atransparent conductive film 708 and an alignment film 709 on atransparent substrate 707. A black mask or color filters may be addedwhen necessary.

After spacers are scattered and a sealing member is printed, a liquidcrystal layer 710 is introduced and sealed, to complete areflection-type liquid crystal panel having the structure shown in FIG.7. The kind of the liquid crystal layer 710 can be selected as desiredin accordance with the operation mode (ECB mode, guest-host mode, or thelike) of the liquid crystal.

FIG. 8 shows, in a simplified manner, an appearance of an active matrixsubstrate constituting a reflection-type liquid crystal panel as shownin FIG. 7. In FIG. 8, reference numeral 801 denotes a silicon substrateon which a thermal oxidation film is formed according to the process ofthe first embodiment. Numeral 802 denotes a pixel matrix circuit; 803, asource driver circuit; 804, a gate driver circuit; and 805, a logiccircuit.

Although in a broad sense the logic circuit 805 includes all logiccircuits that are constituted of TFTs, in this embodiment it meanssignal processing circuits (a memory, a D/A converter, a clockgenerator, etc.) other than those logic circuits to discriminate it fromcircuits conventionally called a pixel matrix circuit or a drivercircuit.

The liquid crystal panel thus formed is provided with FPC (flexibleprint circuit) terminals as external terminals. Liquid crystal panelsthat are commonly called a liquid crystal module is ones provided withFPC terminals.

Embodiment 6

This embodiment is directed to a case of constructing atransmission-type liquid crystal panel by using semiconductor devicesaccording to the first embodiment. This embodiment will be describedwith reference to FIG. 9. Since the basic configuration of thisembodiment is the same as that of the reflection-type liquid crystalpanel of the fifth embodiment, different points will mainly be describedbelow.

A transmission-type liquid crystal panel shown in FIG. 9 is muchdifferent from the reflection-type liquid crystal panel in the structureof a black mask 901. That is, in the transmission type, to increase theaperture ratio, it is important that the area of those portions otherthan a TFT portion and wiring portions which are covered with the blackmask 901 be minimized.

To this end, in this embodiment, a drain electrode 902 is formed so asto overlap with the TFT portion and an auxiliary capacitor is formedbetween the black mask 901 and the drain electrode 902 above the TFTportion. By forming the auxiliary capacitor that occupies a large areaabove the TFT portion, a large aperture ratio can be obtained.

Reference numeral 903 denotes a transparent conductive film as a pixelelectrode. Although ITO is most frequently used as a material of thetransparent conductive film 903, other materials such as tin oxide mayalso be used.

FIG. 23A shows a top view of a pixel structure of the embodiment, inwhich attention is attached to a pixel TFT portion. In FIG. 23A,reference numerals 51 to 55 denote an active layer, a source line, agate line, a drain electrode, and black mask, respectively. Referencenumeral 56 also denotes a contact hole for connecting the drainelectrode 54 to a pixel electrode 57.

The feature of this embodiment resides in that an auxiliary capacitor 59is formed between the drain electrode 54 and the black mask 55 above thepixel TFT portion.

FIG. 23B shows a sectional structure cut along the broken line to A-A′.Same reference numerals are used in FIGS. 23A and 23B. Further, FIG. 24shows a TEM photograph in which the cross section corresponding to FIG.23B is actually photographed.

As described above, the drain electrode 55 is formed so as to beoverlapped with the gate line, and the auxiliary capacitor 59 is formedbetween the facing black mask 55 while sandwiching a dielectric 58. Inthis embodiment, three-layer structure is employed in which a titaniumfilm as the drain electrode 54 is sandwiched between aluminium films.

In this embodiment, the drain electrode 54 is formed, then an interlayerinsulating film of three-layer structure consisting of a silicon nitridefilm, a silicon oxide film, and a acrylic film is formed, and the blackmask 55 is formed thereupon.

In this case, before forming the black mask 55, the acrylic film only inthe region where the auxiliary capacitor 59 will be formed later, isremoved, thereby forming an opening. As a result, only silicon oxide andsilicon nitride are remained at the bottom of the opening, and theinsulating layer of two-layer structure functions as the dielectric 58for the auxialily capacitor 59.

Embodiment 7

This embodiment is directed to a case where the invention is applied towhat is called a silicon-gate TFT in which a conductive silicon film isused as a gate electrode. Since the TFT of this embodiment hasapproximately the same basic structure as that of the first embodiment,only different points will be described below.

Referring to FIG. 10, reference numerals 11-13 denote gate electrodes ofan n-channel TFT, a p-channel TFT, and a pixel TFT, respectively. Thegate electrodes 11-13 are made of an n-type polysilicon film to whichphosphorus or arsenic is added or a p-type polysilicon to which boron orindium is added.

The CMOS circuit may be a dual gate CMOS circuit in which an n-typepolysilicon gate is used in the n-channel TFT and a p-type polysilicongate is used in the p-channel TFT.

Using a silicon film as the gate electrode in the above manner hasadvantages that the heat resistance is high and the silicon film is easyto handle. Further, a salicide structure (including a polycidestructure) can be formed by utilizing reaction with a metal film.

To this end, sidewalls 14-16 are formed after formation of the gateelectrode 11-13. Then, after a metal film (not shown) such as a tungstenfilm or a titanium is formed, metal silicide films 17-19 are formed byperforming a heat treatment. The metal silicide films 17-19 are formedas part of the source and drain electrodes and the gate electrode.

The structure in which a metal silicide film is formed in a self-alignedmanner by using a sidewall or the like in this manner is called thesalicide structure. This structure is effective because good ohmiccontact to the pickup electrodes (source and drain electrodes etc.) canbe obtained.

Embodiment 8

This embodiment is directed to a case of using germanium as a catalystelement in crystallizing an amorphous silicon film. This embodiment willbe described with reference to FIGS. 16A-16C.

First, a quartz substrate is prepared as a substrate 41. An insulatingfilm such as a silicon oxide film may be formed as an undercoat film, ifnecessary.

Then, an amorphous silicon film 42 is formed by low-pressure CVD byusing disilane (Si₂H₆) as a film forming gas (see FIG. 16A). In thisembodiment, the thickness of the amorphous silicon film 42 is set at 75nm.

Then, a step of crystallizing the amorphous silicon film 42 is executed.In this embodiment, germanium is used as a catalyst element foraccelerating crystallization in crystallizing the amorphous silicon film42. In this embodiment, a germanium film 43 is formed on the amorphoussilicon film 42 by plasma CVD.

A germane (GeH₄) gas diluted with hydrogen or helium by a factor of 5 to10 is used as a film forming gas. Then, a germanium film 43 of 1-50 nm(typically 10-20 nm) in thickness can be formed by causing discharge at20-50 mW/cm² at a film forming temperature of 100°-300° C.

Alternatively, the germanium film 43 may be formed by low-pressure CVD.Since a germane gas is very apt to decompose, a germanium film can beformed with easy decomposition of a germane gas at as low a temperatureas about 450° C.

The state of FIG. 16A is thus obtained. Then, the amorphous silicon film42 is crystallized by performing a heat treatment at 450°-650° C.(preferably 500°-550° C.) as shown in FIG. 16B. The reason why the upperlimit temperature is set at 600° C. is that at a temperature higher than600° C. the rate of occurrence of natural nuclei increases to such alevel that they are mixed with crystals that are formed with germaniumatoms as nuclei, to cause disorder in crystallinity.

In the crystallization step, any of furnace annealing, lamp annealing,and laser annealing may be employed. In this embodiment, furnaceannealing is employed with importance attached to the uniformity of aresulting film.

A resulting crystal silicon film (polysilicon film) 44 has superiorcrystallinity in spite of the fact that it has been formed at as low atemperature as about 500° C.

Then, after the residual germanium film on the crystal silicon film 44is removed with a sulfuric acid-hydrogen peroxide solution(H₂SO₄:H₂O₂=1:1), a heat treatment is performed on the crystal siliconfilm 44 at a temperature (800°-1,050° C.) at least higher than theabove-mentioned crystallization temperature (see FIG. 16C).

Where germanium is used as the catalyst element for acceleratingcrystallization of silicon as in this embodiment, there is no particularreason for removing germanium by gettering it. It is considered thatsince germanium is a semiconductor element belonging to the same groupas silicon and they are compatible with each other, germanium does notadversely affect the semiconductor characteristics of silicon.

A crystal silicon film 45 having high crystallinity is formed by theheat treatment step. A thermal oxidation film 46 is formed on thecrystal silicon film 45 by the heat treatment step. The thermaloxidation film 46 can be used, as it is, as a gate insulating film informing a TFT.

The heat treatment may be performed in a state that the germanium filmis left, in which case germanium comes to exist in the film at a highconcentration. In either case, after the heat treatment step, diffusioncauses germanium to exist in the crystal silicon film 45 at aconcentration of 1×10¹⁴ to 5×10¹⁹ atoms/cm³ (typically 1×10¹⁵ to 1×10¹⁶atoms/cm³).

Therefore, the crystal silicon film 45 formed in this embodiment is asemiconductor film that contains many bonds where a silicon atom isreplaced by a germanium atom and is close to a silicon-germaniumsemiconductor that is expressed by Si_(x)Ge_(1-x)(0<X<1).

The manufacturing process of this embodiment has an advantage thatabnormal growth of silicon oxide does not occur at all on the crystalsilicon film 45. That is, abnormal growth of silicon oxide does notoccur with the crystal silicon film 45 that is formed by the process ofthis embodiment even if thermal oxidation is performed in a state thatthe crystal silicon film 45 is in contact with an oxidizing atmosphere.

The present assignee confirmed that where nickel is used as a catalystfor crystallization, abnormal growth of silicon oxide occurs in a laterheat treatment step depending on its conditions. This is due toconcentrated oxidation of nickel silicide existing in the crystalsilicon film. No such abnormal growth occurs in this embodiment.

Further, the heat treatment step of FIG. 16C can remove, almostcompletely, intragrain defects that existed in the crystal silicon film44. The crystal silicon film 44 as crystallized, that is, in the stateof FIG. 16B, has many defects (stacking faults, dislocations, etc.) incrystal grains. However, the crystal silicon film 45 obtained by thestep of FIG. 16C has almost no defects in crystal grains.

The present assignee infers that the absence of defects is caused byphenomena similar to those that were explained in the first embodimentby using the models. However, it is considered that since nickel is notused as a catalyst element, the extinction of defects is mainly owed tothe influence of excess silicon atoms generated by the thermaloxidation.

Although in the process of FIG. 15A the temperature is reduced to theroom temperature after the completion of the crystallization step, thefixing step may be performed by increasing the temperature immediatelyafter the completion of the crystallization step. Such a process canproduce a crystal silicon film having similar crystallinity.

As described above, by employing the process of this embodiment,abnormal oxidation of a crystal silicon film can be prevented and hencethe heat treatment step for the crystal silicon film does not becomeunduly complex. Further, intragrain defects in the crystal silicon filmare removed whereby the crystal silicon film is given very highcrystallinity.

The temperature higher than the crystallization temperature is typically800°-1,050° C. (preferably 850°-900° C.). This embodiment ischaracterized in performing the heat treatment at such a hightemperature. Since it is considered that in this step the thermaloxidation greatly contributes to the reduction of intragrain defects, itis desirable that this step be executed under such conditions as tofacilitate thermal oxidation.

Therefore, in view of the throughput, it is preferable to set the lowerlimit temperature of the heat treatment at 800° C. In view of the heatresistance of the substrate (in this embodiment, the quartz substrate),it is preferable to set the upper limit temperature at 1,050° C.However, since the melting point of germanium is 930°-940° C., it iseven preferable to set the upper limit temperature at 900° C.

It is preferable that the heat treatment atmosphere be an oxidizingatmosphere, it may be an inert atmosphere. The oxidizing atmosphere maybe one of a dry oxygen (O₂) atmosphere, a wet oxygen (O₂+H₂) atmosphere,and an atmosphere containing a halogen element (O₂+HCl or the like).

In particular, if the heat treatment is performed in an atmospherecontaining a halogen element, excess germanium atoms existing betweenthe lattice points of crystal silicon are removed in the form of GeCl₄that is volatile by the gettering effect of the halogen element.Therefore, this is an effective means for obtaining a crystal siliconfilm with less lattice strains.

The crystal silicon film of this embodiment formed by theabove-described manufacturing method has very high crystallinity.

Embodiment 9

Since a TFT according to the invention is formed by using, as its activelayer, a semiconductor thin film that can substantially be regarded as asingle crystal, it exhibits electric characteristics equivalent to thoseof a MOSFET using single crystal silicon. TFTs that were producedexperimentally by the inventors provided the following data.

(1) For each of an n-channel TFT and a p-channel TFT, the subthresholdcoefficient that is an index of the switching performance (quickness ofswitching between on and off operations) of a TFT is as small as 60-100mV/decade (typically 60-85 mV/decade).

(2) The field-effect mobility (μ_(FE)) that is an index of the operationspeed of a TFT is as large as 200-650 cm²/Vs (n-channel TFT; typically250-300 cm²/Vs) or as large as 100-300 cm²/Vs (p-channel TFT; typically150-200 cm²/Vs).

(3) The threshold voltage (V_(th)) that is an index of the drive voltageof a TFT is as small as −0.5 to 1.5 V (n-channel TFT) or as small as−1.5 to 0.5 V (p-channel TFT).

As described above, a TFT produced by the invention has a far superiorswitching characteristic and high-speed operation characteristic. Thisenables integrated circuits such as an LSI to be constructed by usingTFTs rather than MOSFETs that are conventionally used.

Further, by utilizing the advantages of the TFT in which a thin film isused, a three-dimensional semiconductor device (semiconductor circuit)can be constructed.

FIGS. 11A and 11B show examples of three-dimensional semiconductorcircuits using TFTs of the invention. FIG. 11A shows a three-dimensionalcircuit in which TFTs are formed in the bottom layer and an image sensoris formed in the top layer. FIG. 11B shows a three-dimensional circuitin which TFTs are formed in each of the bottom layer and the top layer.

In FIG. 11A, a photoelectric conversion layer 21 may be made of anamorphous silicon film or the like. A top electrode (transparentconductive film) 22 is formed on the photoelectric conversion layer 21,to thereby constitute a photodetecting section for receiving light andconverting it into an electrical signal.

Since the manufacturing process for producing the TFTs is the same as inthe first embodiment, it is not described here. The multi-layers formingtechnique for constructing the three-dimensional circuit may be a knownone. However, in forming the top TFT layer, it is necessary to considerthe heat resistance of the TFTs in the bottom layer.

For example, the three-dimensional circuit may have a configuration inwhich TFTs according to the invention are used in the bottom layer andconventional, low-temperature formation TFTs are used in the top layer.Or the three-dimensional circuit may have a configuration in which TFTsmade of a highly heat-resistant material are used in the bottom layerand TFTs according to the invention are used in the top layer.

A further alternative may be such that the image sensor in the top layeris composed of only the photodetecting section and photodetectingsection is controlled by the TFTs in the bottom layer.

In FIG. 11B, the bottom layer is the TFT layer using a silicon-gatestructure and the top layer is the TFT layer in which a silicon-gatestructure or a metal film (for instance, a film having aluminum as themain component) is used as a gate electrode. The manufacturing processof the TFTs of FIG. 11B is not described either.

Also in the configuration of FIG. 11B, the top TFT layer needs to beformed with sufficient consideration given to the heat resistance of theTFTs in the bottom layer.

In each case of FIGS. 11A and 11B, employment of the following steps isdesirable. First, a thick interlayer insulating film 23 or 24 is formedafter the formation of the TFTs in the bottom layer. Then, after theinterlayer insulating film 23 or 24 is planarized by polishing such aschemical mechanical polishing (CMP), the TFTs in the top layer areformed.

By forming a three-dimensional semiconductor circuit by using TFTs ofthe invention, a semiconductor circuit having a wide variety offunctions can be constructed. In this specification, the term“semiconductor circuit” means an electric circuit forcontrolling/converting an electrical signal by utilizing thesemiconductor characteristics.

It is also possible to construct a LCD driver circuit, a high-frequencycircuit (MMIC: microwave module IC) for portable equipment, etc. byusing TFTs of the invention. That is, the use of TFTs of the inventionmakes it possible to construct IC chips and LSI chips by using TFTs.

Embodiment 10

In addition to a liquid crystal display device, other electro-opticaldevices such as an active matrix EL (electroluminescence) display deviceand an EC (electrochromic) display device as well as an image sensor anda CCD can be constructed according to the invention.

The term “electro-optical device” means a device for converting anelectrical signal to an optical signal, or vice versa.

Embodiment 11

In this embodiment, examples of electronic apparatuses (applicationproducts) using an electro-optical device according to the inventionwill be described with reference to FIGS. 12A-12F. The term “electronicapparatus” means a product incorporating a semiconductor circuit and/oran electro-optical device.

Examples of electronic apparatuses to which the invention can be appliedare a video camera, an electronic still camera, a projector, ahead-mounted display, a car navigation apparatus, a personal computer,and portable information terminals (a mobile computer, a cellulartelephone, a PHS (personal handyphone system) telephone, etc.).

FIG. 12A shows a cellular telephone, which is composed of a main body2001, a voice output section 2002, a voice input section 2003, a displaydevice 2004, manipulation switches 2005, and an antenna 2006. Theinvention can be applied to the voice output section 2002, the voiceinput section 2003, the display device 2004, etc.

FIG. 12B shows a video camera, which is composed of a main body 2101, adisplay device 2102, a sound input section 2103, manipulation switches2104, a battery 2105, and an image receiving section 2106. The inventioncan be applied to the display device 2102, the sound input section 2103,the image receiving section 2106, etc.

FIG. 12C shows a mobile computer, which is composed of a main body 2201,a camera section 2202, an image receiving section 2203, a manipulationswitch 2204, and a display device 2205. The invention can be applied tothe camera section 2202, the image receiving section 2203, the displaydevice 2205, etc.

FIG. 12D shows a head-mounted display, which is composed of a main body2301, display devices 2302, and a band section 2303. The invention canbe applied to the display device 2302.

FIG. 12E shows a rear projector, which is composed of a main body 2401,a light source 2402, a display device 2403, a polarizing beam splitter2404, reflectors 2405 and 2406, and a screen 2407. The invention can beapplied to the display device 2403.

FIG. 12F shows a front projector, which is composed of a main body 2501,a light source 2502, a display device 2503, an optical system 2504, anda screen 2505. The invention can be applied to the display device 2503.

As described above, the invention has an extremely wide applicationrange and can be applied to electronic apparatuses of every field.Further, the invention can be applied to any product that requires anelectro-optical device or a semiconductor circuit.

The invention enables formation of a semiconductor thin film having suchcrystallinity as to substantially be regarded as a single crystal, andmakes it possible to realize, by using such a semiconductor thin film, aTFT exhibiting high performance that is equivalent to or even higherthan the performance of a MOSFET formed on a single crystal.

A semiconductor circuit and an electro-optical device that are formed byusing TFTs of the above kind, and an electronic apparatus incorporatingsuch a semiconductor circuit or electro-optical device have extremelyhigh performance and are products that are superior in functionality,portability, and reliability.

1. A semiconductor device comprising: a first thin film transistorcomprising a first semiconductor film over an insulating surface; aninsulating film over the first thin film transistor; a secondsemiconductor circuit comprising a second semiconductor film over theinsulating film; a wiring electrically connected to the secondsemiconductor circuit; a photoelectric conversion layer on the wiring;and an electrode on the photoelectric conversion layer, wherein at leastone of the first and second semiconductor films is a crystalline siliconfilm, wherein each of diffraction spots of an electron beam diffractionpattern of the one of the first and second semiconductor films issubstantially circular, and wherein a ratio of a minor-axis length to amajor-axis length of each of the diffraction spots is in a range of 1/1to 1/1.5.
 2. An electronic apparatus according to claim 1, wherein theelectronic apparatus is selected from the group consisting of a videocamera, an electric still camera, a projector, a head-mounted display, acar navigation apparatus, a personal computer, a mobile computer, and acellular phone.
 3. A semiconductor device comprising: a first thin filmtransistor comprising a first semiconductor film over an insulatingsurface; an insulating film having a planarized surface over the firstthin film transistor; a second semiconductor circuit comprising a secondsemiconductor film over the planarized surface; a wiring electricallyconnected to the second semiconductor circuit; a photoelectricconversion layer on the wiring; and an electrode on the photoelectricconversion layer, wherein at least one of the first and secondsemiconductor films is a crystalline silicon film, wherein each ofdiffraction spots of an electron beam diffraction pattern of the one ofthe first and second semiconductor films is substantially circular, andwherein a ratio of a minor-axis length to a major-axis length of each ofthe diffraction spots is in a range of 1/1 to 1/1.5.
 4. An electronicapparatus according to claim 3, wherein the electronic apparatus isselected from the group consisting of a video camera, an electric stillcamera, a projector, a head-mounted display, a car navigation apparatus,a personal computer, a mobile computer, and a cellular phone.
 5. Asemiconductor device comprising: a first thin film transistor comprisinga first gate electrode and a first semiconductor film over an insulatingsurface; an insulating film over the first thin film transistor; asecond semiconductor circuit comprising a second gate electrode and asecond semiconductor film over the insulating film; a wiringelectrically connected to the second semiconductor circuit; aphotoelectric conversion layer on the wiring; and an electrode on thephotoelectric conversion layer, wherein the first gate electrodecomprises a silicon and the second gate electrode comprises a silicon,wherein at least one of the first and second semiconductor films is acrystalline silicon film, wherein each of diffraction spots of anelectron beam diffraction pattern of the one of the first and secondsemiconductor films is substantially circular, and wherein a ratio of aminor-axis length to a major-axis length of each of the diffractionspots is in a range of 1/1 to 1/1.5.
 6. An electronic apparatusaccording to claim 5, wherein the electronic apparatus is selected fromthe group consisting of a video camera, an electric still camera, aprojector, a head-mounted display, a car navigation apparatus, apersonal computer, a mobile computer, and a cellular phone.
 7. Asemiconductor device comprising: a first thin film transistor comprisinga first gate electrode and a first semiconductor film over an insulatingsurface; an insulating film over the first thin film transistor; asecond semiconductor circuit comprising a second gate electrode and asecond semiconductor film over the insulating film; a wiringelectrically connected to the second semiconductor circuit; aphotoelectric conversion layer on the wiring; and an electrode on thephotoelectric conversion layer, wherein the first gate electrodecomprises a silicon and the second gate electrode comprises a metalfilm, wherein at least one of the first and second semiconductor filmsis a crystalline silicon film, wherein each of diffraction spots of anelectron beam diffraction pattern of the one of the first and secondsemiconductor films is substantially circular, and wherein a ratio of aminor-axis length to a major-axis length of each of the diffractionspots is in a range of 1/1 to 1/1.5.
 8. An electronic apparatusaccording to claim 7, wherein the electronic apparatus is selected fromthe group consisting of a video camera, an electric still camera, aprojector, a head-mounted display, a car navigation apparatus, apersonal computer, a mobile computer, and a cellular phone.
 9. Asemiconductor device according to claim 7, wherein the metal filmcomprises an aluminum.